This could easily be a continuation bet of a missed AK or AQ. How do you know if you should continue with the hand or do you normally just give it up?

Party Poker No-Limit Hold'em, $0.5 BB (9 handed)

Button ($23.5)
SB ($54.35)
BB ($118.4)
UTG ($56.65)
UTG+1 ($50.25)
MP1 ($59.25)
MP2 ($65.1)
Hero ($51.75)
CO ($48.75)

Preflop: Hero is MP3 with , .
UTG calls $0.50, 1 fold, MP1 raises to $3, 1 fold, Hero calls $3, 4 folds, UTG folds.

Flop: ($7.25) , , (2 players)
MP1 bets $4, Hero folds.

Final Pot: $11.25

Results in white below:
No showdown. MP1 wins $11.25.

It could be, but only a calling station or maniac is finding out. If he lead for half pot, I would say you can pump it up with 77 but that is pretty close to full pot if I remember correctly. Even if it is a continuation bet with something like 99 TT you are still in a bad spot. Knowing your opponents betting patterns makes it much easier to play.